Chlorine-Rich Silicic Fluids During the Onset of Fibrous Diamond Growth in the Democratic Republic of the Congo

High-density fluid (HDF) micro-inclusions commonly found in fibrous diamonds offer a unique opportunity to explore the metasomatic events in the lithospheric mantle, during which the micro-inclusions and their host diamonds formed. It has been suggested that saline and silicic HDFs are immiscible due to the absence of intermediate compositions between these two end-members. In the present study, however, SiO₂- and Cl-rich HDFs with compositions parallel to the saline–silicic array were identified at the core–coat boundaries of coated diamonds from the Democratic Republic of the Congo (DRC). HDFs in these diamonds exhibit a consistent evolution from intermediate saline–silicic compositions at the core–coat boundary to more silicic ones in the inner layer, and then to more carbonatitic ones in the outer layer. Nitrogen aggregation thermochronology, combined with diamond–inclusion isomekes and orthopyroxene thermobarometries, yields P–T ranges of 4.8–6.6 GPa and 1060–1200 °C for the cores, and 4.1–6.5 GPa and 970–1020 °C for the coats and included HDFs. These ranges correspond respectively to the higher and lower parts of the P–T range for octahedral diamonds from DRC. Compositional characteristics of these HDFs and features of different growth layers indicate that the HDF evolution spanning over a significant range of saline–silicic–carbonatitic compositions, along with the growth of different fibrous layers is the result of the continuous variation of source H₂O/CO₂ ratios during a metasomatic event. Cl-rich silicic fluids are possible to form in the mantle under high source H₂O/CO₂ ratios, which may not be conducive to diamond crystallization. This study provides new insights into the features and evolution of mantle fluids and reveals potential connections between diamond crystallization and fluid composition.